1. Field of the Invention
The present invention relates to a method for determining a laser threshold of a laser diode, which is operated by a drive circuit as a function of a first signal in a first feedback and a second signal in a second feedback, the first feedback being supplied via an optical coupling by an optical output signal of the laser diode.
The invention relates furthermore to a drive circuit, which determines the laser threshold of a laser diode, wherein the drive circuit generates a drive signal and the laser diode an output signal, dependent on the drive signal, with a first feedback and a second feedback, the first feedback being set up to supply a portion of the output signal via an optical coupling.
2. Description of the Background Art
A method and drive circuit are known from U.S. Pat. No. 5,260,955. According to this publication, the optical power/drive current characteristic of a laser diode has a lower, nonlinear section of lower drive currents and optical powers. Furthermore, the characteristic has an upper section of higher drive currents and optical powers, in which the optical power depends approximately linearly on the drive current. The linear section in the patent is called a lasing region and the nonlinear region is called a light emitting or spontaneous emission region. Both sections are separated by a value of the drive current, which is also called the laser threshold. Knowing the laser threshold is helpful for controlling the laser diode.
To determine the laser threshold two feedbacks are provided in U.S. Pat. No. 5,260,955, which are connected by a common electrical connection node to a photodiode. A first feedback serves to regulate the power of the laser diode during laser diode operation. The second feedback serves to determine the laser threshold. To that end, the signal in the second feedback is compared with a fixed threshold value. As long as the threshold value is not exceeded, an increase in the laser diode drive current occurs successively. If the threshold value, which correlates with a specific optical power received by the photodiode, is exceeded, the associated drive current is maintained with no further changes. During operation, the second feedback then supplies this laser threshold/drive current, whereas a reference signal generator supplies control signals for the drive current contributions above the laser threshold. These are stabilized with the first feedback.
In prior-art arrangements of drive circuits and laser diodes, the stabilization occurs in the following manner: The first feedback is connected to a feedback signal input of a differential amplifier and receives part of the emitted laser diode optical power via the photodiode. A reference signal, which controls the optical power of the laser diode, is supplied furthermore to the differential amplifier. If the laser diode emits a relatively high optical power, the photodiode supplies a high photocurrent to the feedback signal input of the differential amplifier, which makes the output signal of the differential amplifier smaller and thereby reduces the laser diode optical power. Accordingly, a relatively low emitted optical power leads to an increase in the difference and thereby to an increase in the optical power. The feedback thereby closes a control loop by which a stable optical power becomes established in the built-up state.
The laser thresholds for driving individual laser diodes can also be individually different in laser diodes of the same type and in addition change due to aging influences. In a laser diode type serving as an example here, the individual laser threshold of an individual laser diode can be, for example, within a range of drive current intensities between 10 mA and 50 mA. To be able to generate defined output signals with defined drive signals, it is therefore helpful to know the individual laser threshold of a laser diode. This applies very generally also to other components that have an activation threshold, or to arrangements comprising a differential amplifier and a laser diode. In this type of arrangement, the drive current intensity, at which a laser effect begins in the laser diode, depends in addition on the offset current of the amplifier. If an output signal of a laser diode is indicated below, this usually refers to an optical power above the laser threshold or activation threshold.
For the greatest possible control for the optical power of the laser diode and thereby the output signals of the laser diode, it is desirable to know the tolerance pertaining to the laser threshold.
In addition, drive current sources, for example, have various examples of the aforementioned differential amplifier, frequently also dispersion in their input/output signal relation, which are to be compensated for a defined function of the component by adjustment of the differential amplifier. This adjustment can occur the more accurately, the better the operating condition of the differential amplifier during adjustment coincides with the operating conditions during later operation. Because drive current intensities of the magnitude of the laser threshold are to be provided during later operation, the adjustment should occur as closely as possible to the laser threshold. For this reason as well, it is desirable to know the laser threshold.
In the method disclosed in U.S. Pat. No. 5,260,955, to determine the laser threshold, the drive signal passes through a certain signal bandwidth and in so doing, an activity in the signal path behind the optical coupling between the laser diode and photodiode is determined by a threshold value comparison. In practice, this type of feedback having an optical coupling reacts especially very sensitively to measuring actions in laser diodes. A sufficient reliability and accuracy of the obtained results are therefore not readily assured.